1. Field of the Invention
The present invention relates to a method and apparatus for handling transmission time interval (TTI) bundling retransmission, and more particularly, to a method and apparatus for handling retransmission of a TTI bundle in a user equipment (UE) of a wireless communication system, so as to avoid improper Hybrid Automatic Repeat Request (HARQ) retransmissions.
2. Description of the Prior Art
Long Term Evolution wireless communication system (LTE system), an advanced high-speed wireless communication system established upon the 3G mobile telecommunication system, supports only packet-switched transmission, and tends to implement both Medium Access Control (MAC) layer and Radio Link Control (RLC) layer in one single communication site, such as in Node B (NB) alone rather than in NB and RNC (Radio Network Controller) respectively, so that the system structure becomes simple.
In LTE, a technique of transmission time interval (TTI) bundling is introduced for improving uplink coverage according to the prior art. TTI bundling is performed by repeatedly coding and transmitting a same Transport block in a set of consecutive subframes (e.g. TTI), and those repeatedly transmitted packets are named a TTI bundle. UEs in cell boundary utilizing TTI bundling can reduce transmission delay and signaling of control channels, and enhance reliability and accuracy of transmission, such that LTE uplink coverage can be improved.
According to current specifications, TTI bundling is characterized as below:
(1) The same HARQ process is used for each transmission of a TTI bundle.
(2) TTI bundling is switched on/off per UE with higher layer signaling, e.g. Radio Resource Control (RRC) signaling. When switched on, TTI bundling would apply to all uplink transmissions using Physical Uplink Shared Channel (PUSCH).
(3) A TTI bundle is treated as a single resource, i.e., a single grant and a single HARQ feedback (e.g. acknowledgement signal ACK or non-acknowledgement signal NACK) is used for each TTI bundle.
It is worth noting that the timing relationship between the last transmission of a TTI bundle and its HARQ feedback is the same as the normal HARQ operation (i.e. non-bundled operation), and that the HARQ Round Trip Time (RTT) for TTI bundling is twice the RTT of normal HARQ operation. That is to say, if the last transmission of a TTI bundle occurs at TTI n, the HARQ feedback is received at TTI (n+4); and if a first transmission of a TTI bundle occurs at TTI k, retransmission of the TTI bundle starts at TTI (k+2*HARQ_RTT), where HARQ_RTT represents the round trip time of the normal HARQ operation. In addition, the size of a TTI bundle, i.e. the number of TTIs required to transmit the TTI bundle, is fixed as 4.
According to 3GPP meeting document R2-083726, the number of HARQ processes is reduced from 8 to 4 when TTI bundling is configured. In other words, the number of HARQ processes changes when the UE activates or deactivates the TTI bundling mode. As for detailed HARQ operations for TTI bundling, please refer to related meeting document R2-083724.
As indicated in the above meeting document, the retransmission of a TTI bundle is also a TTI bundle. Besides, within the TTI bundle, HARQ retransmissions are non-adaptive and shall be performed without waiting for feedback from previous transmissions. Compared to new transmissions and adaptive retransmissions which are performed on the resource indicated on PDCCH, a non-adaptive retransmission is performed on the same resource as was used for the last transmission. Related HARQ operation is known by those skilled in the art, and is not narrated herein.
Thus, if the UE does not receive an uplink grant for a current transmission opportunity on PDCCH, the HARQ process of the prior art shall generate a non-adaptive retransmission for the following events:
(1) TTI bundling is not configured, and the last feedback for this HARQ process is an HARQ NACK;
(2) TTI bundling is configured, the current transmission opportunity is for a first transmission within a TTI bundle, and the last received feedback for this HARQ process is an HARQ NACK; or
(3) TTI bundling is configured, and the current transmission opportunity is not for a first transmission within a TTI bundle.
That means, when the UE does not receive an uplink grant for a first retransmission opportunity of a retransmission bundle, a non-adaptive retransmission is generated for the first transmission opportunity of the retransmission bundle if the last received feedback for this HARQ process is an HARQ NACK. For the following transmission opportunities of the retransmission bundle, there is no need to check the last received feedback for this HARQ process i.e. non-adaptive retransmissions are always generated.
As a result, if the last received feedback for this HARQ process is an HARQ ACK, the UE shall not generate anon-adaptive retransmission for the first transmission opportunity of the bundle retransmission. But non-adaptive retransmissions are still generated for the following transmission opportunities of the retransmission bundle. Since the HARQ ACK means that the network has received the transport block (TB), there is no need for UE to perform any non-adaptive retransmission unless a PDCCH with uplink grant is received. Otherwise, it may cause interference to other UE's transmission.
On the other hand, if the UE receives an uplink grant for a first retransmission opportunity of a TTI bundle, the UE shall request the HARQ process to generate a new transmission or an adaptive retransmission. In this case, the first transmission of the TTI bundle is generated according to an uplink grant received on PDCCH, while following transmissions of the TTI bundle are still generated in a non-adaptive retransmission way.